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  Java Practices -> Class for constants
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<div class='page-title'>Class for constants</div>

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<br>Creating a class whose sole job is to define widely-used constants
is simple.

<p><b>Example</b>
<br>
<PRE>

<span class='keyword'>package</span> hirondelle.web4j.util;

<span class='comment'>/**
 Collected constants of general utility.

&lt;P&gt;All members of this class are immutable. 

 &lt;P&gt;(This is an example of 
 &lt;a href='http://www.javapractices.com/Topic2.cjp'&gt;class for constants&lt;/a&gt;.)
*/</span>
<span class='keyword'>public</span> <span class='keyword'>final</span> <span class='keyword'>class</span> Consts  {

  <span class='comment'>/** Opposite of {@link #FAILS}.  */</span>
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>final</span> <span class='keyword'>boolean</span> PASSES = <span class='keyword'>true</span>;
  <span class='comment'>/** Opposite of {@link #PASSES}.  */</span>
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>final</span> <span class='keyword'>boolean</span> FAILS = <span class='keyword'>false</span>;
  
  <span class='comment'>/** Opposite of {@link #FAILURE}.  */</span>
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>final</span> <span class='keyword'>boolean</span> SUCCESS = <span class='keyword'>true</span>;
  <span class='comment'>/** Opposite of {@link #SUCCESS}.  */</span>
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>final</span> <span class='keyword'>boolean</span> FAILURE = <span class='keyword'>false</span>;

  <span class='comment'>/** 
   Useful for {@link String} operations, which return an index of &lt;tt&gt;-1&lt;/tt&gt; when 
   an item is not found. 
  */</span>
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>final</span> <span class='keyword'>int</span> NOT_FOUND = -<span class='literal'>1</span>;
  
  <span class='comment'>/** System property - &lt;tt&gt;line.separator&lt;/tt&gt;*/</span>
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>final</span> String NEW_LINE = System.getProperty(<span class='literal'>"line.separator"</span>);
  <span class='comment'>/** System property - &lt;tt&gt;file.separator&lt;/tt&gt;*/</span>
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>final</span> String FILE_SEPARATOR = System.getProperty(<span class='literal'>"file.separator"</span>);
  <span class='comment'>/** System property - &lt;tt&gt;path.separator&lt;/tt&gt;*/</span>
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>final</span> String PATH_SEPARATOR = System.getProperty(<span class='literal'>"path.separator"</span>);
  
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>final</span> String EMPTY_STRING = <span class='literal'>""</span>;
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>final</span> String SPACE = <span class='literal'>" "</span>;
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>final</span> String TAB = <span class='literal'>"&#092;t"</span>;
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>final</span> String SINGLE_QUOTE = <span class='literal'>"'"</span>;
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>final</span> String PERIOD = <span class='literal'>"."</span>;
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>final</span> String DOUBLE_QUOTE = <span class='literal'>"&#092;""</span>;

  <span class='comment'>// PRIVATE //
</span>
  <span class='comment'>/**
   The caller references the constants using &lt;tt&gt;Consts.EMPTY_STRING&lt;/tt&gt;, 
   and so on. Thus, the caller should be prevented from constructing objects of 
   this class, by declaring this private constructor. 
  */</span>
  <span class='keyword'>private</span> Consts(){
    <span class='comment'>//this prevents even the native class from 
</span>    <span class='comment'>//calling this ctor as well :
</span>    <span class='keyword'>throw</span> <span class='keyword'>new</span> AssertionError();
  }
}
 
</PRE>
<br>
The caller refers to these constants using static references, such as <tt>Consts.EMPTY_STRING</tt>, <tt>Consts.SUCCESS</tt>, and so on.
Since the caller will never need to create a <tt>Consts</tt> object, a <a href='TopicAction79e9-2.html'>private constructor</a> is used to enforce that policy.
The members of such a constants class are excellent candidates for a <a href="TopicActionda46-2.html">static import</a>.

<P>It is also possible to <a href="TopicActiona166-2.html">use an interface</a> to
bring constants into scope. Many argue strongly that such a practice is a strange
use of interfaces, which are intended to state the services provided by a class. 
<br>
<br>

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<div class='topic-section'>See Also :</div>
<div class='main-body'>
 
  
  <a href='TopicActiona166-2.html?Id=32'>Interface for constants</a> <br>
 
  
  <a href='TopicAction79e9-2.html?Id=40'>Private constructor</a> <br>
 
  
  <a href='TopicActionda46-2.html?Id=195'>Use static imports rarely</a> <br>
 
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